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The present invention relates to a coil driver, bridge circuit for a direct current motor. Specifically, it relates to bridge circuits for providing the excitation current to the field coils in an electronically commutated D.C. motor.
An electronically commutated, brushless, D.C. motor can have a rotatable permanent magnet rotor, a stator that carries field coils, coil driver circuits, and coil drive control timing circuitry for controlling the operating state of each coil driver circuit for thereby implementing the sequential excitation of the motor field coils. A brushless D.C. motor eliminates or reduces the disadvantages inherent in the mechanical structure of a mechanically commutated motor, i.e., a motor with a commutator and brushes. Specifically, RFI (radio frequency interference) losses and EMI (electromagnetic induction) losses are reduced or eliminated. Also, the brush maintenance factors and the armature maintenance factors are eliminated. Moreover, the power consumption, attributed to armature-brush arcing, is also eliminated.
Typically, a motor electronic timing circuit, which incorporates active electronic components, has been used to provide and/or control the excitation of each motor coil, by controlling the operating state of each, respective, coil driver circuit. In the motor, each coil driver circuit is connected directly between a power supply and each motor coil. The operating state of the coil driver circuit is controlled by control pulses generated by the electronic timing circuit. The electronic timing of the generation of control pulses to each coil driver circuit can be synchronized with rotor position, wherein monitoring and/or feedback circuitry is incorporated, including the use of optical position sensors or magnet position sensors. The power supply and the control pulse generating, electronic timing circuit are sensitive to reverse voltage and voltage spike conditions which can be generated in a coil portion of the circuit network, under electrical fault conditions, rotor stall conditions, and other effects. Each coil driver circuit is a bridge circuit connecting the control portion of the network to the load (coil) portion of the network. It is desirable for bridge coil driver circuits to incorporate reverse voltage and voltage spike protection, thereby isolating the sensitive power supply circuitry and sensitive electronic control circuitry.
Pulse width modulation (PWM) is used in the electronic timing circuit to control each bridge coil driver circuit. The width of these pulses affects the instantaneous torque developed by the coil excitation in the motor, and the pulse rate affects the motor speed. The duty cycle of the generated drive pulses (pulse width) is controlled electronically.
The sensitive circuitry used to generate the PMW control pulses cannot be subjected to over voltage and over current spikes caused by motor faults and stalls, nor can it be subjected to spikes often generated when the excitation drive current to a field coil is stopped. Such circuit conditions can burnout this delicate pulse drive circuitry. Moreover, such sensitive circuitry cannot normally carry (pass through) the power needed to excite of the motor field coils, without again burning out. Bridge circuits, whose operation is controlled by the PWM output from the electronic timing circuit, can be used to power the motor field coils directly.
The use of turn off, turn on, xe2x80x9csnubber circuitsxe2x80x9d as a part of a coil bridge circuit, to protect power electronics from overvoltage or overcurrent conditions, is not new. These snubber circuits bridge the connection between the electronic timing circuit and the motor (phase) field coils that are excited. Snubber circuits are devices that limit such over voltage and over current transients that can occur during or immediately after the deactivation of a field coil. These snubber circuits have taken one of two forms, dissipative snubbers and regenerative snubbers.
Dissipative snubbers are circuits which shunt such overvoltage and overcurrent transients to ground. The most common configuration for a dissipative snubber is a Resistor-Capacitor-Diode circuit (RCD) connected between one end of a field coil and ground, with the field coil excitation xe2x80x9cswitchxe2x80x9d connected in parallel with the RCD snubber. This device has proven to be somewhat unsatisfactory in unipolar brushless D.C. motors where low voltage, high current is used. In a unipolar brushless D.C. motor, the current in the coil windings flows in one direction through the coils from a D.C. source to ground.
RCD snubbers have been used in low side drive or boost converter type motor drives. With high side drive configurations, these elements are replaced by diodes connected to the positive voltage. A high side drive is where the excitation switch is connected between the voltage supply (power supply) and the load (field coil) being excited. Some high power, low voltage type switching devices have been used in such snubber. Examples are MOSFETs (metal oxide semiconductor field effect transistors). Reverse protection diodes are used to protect motor (phase) field coils from shorting out. Other diodes provide reverse protection to MOSFET containing circuitry because of the MOSFET circuit source to drain diodes that become forward biased during a reverse polarity condition.
Regenerative snubbers return the energy stored in snubber circuit elements back to the positive voltage supply. Many of these have used transformers, or active elements such as D.C. power supplies (batteries). These prior regenerative snubbers have been used to maintain a constant voltage across the switches in order to prevent a braking of the motor through back electromotive force (EMF); and have also been used to return excess energy stored in the motor field coils back to the positive voltage supply. The elimination of the motor braking action reduces the previously induced voltage spikes due to braking EMF.
Markaran (U.S. patent application Ser. No. 2001/0000293 A1) shows and actively controlled regenerative snubber (power output circuit) for a unipolar brushless D.C. motor. Markaran""s circuitry has an output bridge snubber circuit connected to each motor phase (field) coil. The Markaran bridge circuit includes the series connection of a coil and a capacitor in parallel with a motor field coil, and a cross-connected diode, from the grounded side of the coil to the intermediate point of this series LC circuit. The Markaran bridge circuit maintains a constant voltage across its switches (which are each implemented by transistor devices) to prevent avalanching (i.e., breaking down of the transistor switch) which would result in the braking of the motor through conduction of motor back EMF. Markaran""s concern is minimizing switching losses. Markaran operates his bridge switches at a constant frequency xe2x80x9cfcxe2x80x9d.
Upon opening the Markaran field coil excitation switch S1, the residual energy in the Markaran field coil L begins to discharge through the snubber diode D1 and into the capacitor C connected to ground. This limits the voltage drop across the coil activation switch S1 once it is opened. Once the voltage on the Markaran capacitor C reaches a pre-selected threshold voltage, his second switch S2 is closed and the capacitor C begins to discharge through the inductor L2. Markaran""s stated purpose for the inductor L2 is to slow the discharge of the capacitor C and to ensure that the capacitor is discharged in a sinusoidal fashion for harmonic minimization of the discharge current. The value of the snubber diode generally locks in the effective voltage range of the circuit, which is then limited to that value.
Alvaro, et al. (U.S. Pat. No. 6,239,565 B1) show an output bridge circuit for exciting two phase (field) coils W under the control of signals from a driver circuit DC. Alvaro activates a coil excitation switch SW to excite a desired field coil W. The coil excitation current is to flow from a supply to ground. Alvaro inserts a MOSFET in his ground leg leading from each switch W and controls the conducting state of this MOSFET with signals from the driver circuit DC. A diode bridge RB is used on the power supply to protect from reverse spikes. Alvaro includes an RC (resistor-capacitor) voltage protection circuit PC on the control line to the MOSFET. This RC circuit is used to turn on a transistor T, to shunt to ground the activation voltage from the driver circuit to the MOSFET, when the signal from the driver circuit remains high for a period longer than the time constant of the RC circuit.
The Alvaro capacitor in the RC circuit PC will also charge when the rotor is suddenly stopped due to a fault condition. The same time constant will shut off the MOSFET and therefore the excitation current to the field coils. This protects the motor from over heating burnout.
While prior art bridge circuits may provide over voltage and/or over current protection, they generally do not consider dissipating the energy present in a field coil back to the power supply, or if they do, they do not take into consideration noise reduction, i.e., the reduction of electromagnetic interference, nor cross-conduction and other types of energy losses present with plural inductor circuits.
An object of the present invention is to provide a bridge circuit for driving D.C. motor coils that provides transistor over voltage protection.
Another object is that such bridge circuit operates so that excess coil energy, i.e., the residual coil energy at de-energizing time, is captured (stored) and re-used (returned to the supply).
A further object for such bridge circuit is that it minimizes electromagnetic interference.
An additional object for such bridge circuit is that it avoids or significantly reduces cross-conduction and other similar circuit losses.
An additional further object for such bridge circuit is that it minimizes biasing currents and reduces power supply output transistor overheating.
The objects of the present invention are realized in a D.C. motor, H-type bridge, coil driver circuit connected to a power supply, and which bridge coil driver circuit operates to excite a phase (a field coil) in a brushless, electronically commutated D.C. motor. An individual bridge coil driver circuit is connected to power a respective one of each of the motor field coils. The bridge circuit is structured to cause the lagging, residual, excess current in a field coil to be routed back to the power supply. Current flow through a coil is bi-lateral and is caused to flow, according to the instantaneous phase relation of the coil to the motor rotor polarity, under control of known motor electronic timing pulses, such as pulse width modulated (PWM) pulses. The orientation of the direction of flow of residual coil current remains constant with excitation current.
This H-type bridge, coil driver circuit contains a plurality of transistor switches, one positioned operationally in each leg segment of each side of the circuit""s H shape. The leg segment transistor switches are operationally coupled, as cross-diagonal leg segment coordinated switches, with the cross-diagonal pairs being in paired into, i.e., coupled into, exclusive conducting states. The operating states of these transistor switches are manipulated under the control of the PWM input signals. The bi-lateral current flow through the motor coil is thereby achieved for greater power and efficiency.
These transistor switches are connected into series circuit connection, with each series circuit forming a leg segment of the H-shaped circuit. A motor field coil is connectable as the cross leg of the H-shaped circuit, i.e., between the side legs of the H, at the midpoint node between each series connected switch, side leg pair. A respective end of each side leg series-connected switch circuit is connected to the power supply, while the opposite end of each side leg series-connected switch circuit is connected to ground, either directly or through other circuit components.
An alternate circuit path component for each lower leg segment switch is a diode connected in parallel across each lower leg segment transistor switch, which itself has one end connected to the motor coil node and the other end connected to ground. A second alternate circuit path component for each upper leg segment switch is a diode connected from the respective upper leg segment switch""s motor coil node, in series with an energy storage device, such as a capacitor, connected to ground. The energy stored in this capacitor can drain to the power supply through a resistor connection there between. Reverse polarity protection may also be included on the power supply feed line.
Electromagnetic interference is reduced, and any cross-conduction created by same leg switches, being on at the same time, is prevented by cross coupling the switches through biasing resistors. Biasing current and power transistor switch heat dissipation are both reduced by incorporating composite transistor switch pairs for the transistor switches.
In operation, a first cross-diagonal coupled switch pair is turned on while the second is off. Then the second cross-diagonal coupled switch pair is on while the first is turned off. With the first switch pair turned on, current flows through the motor coil in a first direction. With the second switch pair turned on, current flows through the motor coil in a second/opposite direction. The exception to this operation is that for a short period after the transition, between conducting states of the switch pairs, there is a voltage increase at the previous coil current flow exit node, due to the lagging nature of the coil current, and until this excess energy is dissipated, current wants to flow through the coil in the previously flowing direction. This current is allowed to flow in that direction through the alternate circuit path to charge the capacitor. During this short transition period no current is drawn from the power supply. After this period the capacitor drains towards the power supply and thereafter, normal current flow occurs from the power supply, in the now established second/opposite direction.